\documentclass{article}
\usepackage{pagecolor}%
\usepackage{tikz}%
\begin{document}
\pagecolor{red!30}
\begin{tikzpicture}
\tikzset{mwe/.style={inner sep=1.15ex, text depth=0ex, text height=1ex, text width=0.5em, minimum size=0.5em}}%

\node (A) [mwe] at (0,0) {A};%
\node (B) [mwe] at (4,0) {B};%

\node (C) [mwe] at (0,4) {C};%
\node (D) [mwe] at (2,-4) {D};%

\path (A) -- node[text height=1.25ex] (AB) {ab} (B);
\path (C) -- node[text height=1.25ex] (CD) {c} (D);

\draw[line width=0.25mm,-]  (A) -- (CD) -- (AB) --  (B);%
\draw[line width=0.25mm,-]  (C) -- (CD) -- (D);%

\end{tikzpicture}

\end{document}